Various processes have been proposed for reducing the content of sulfur compounds in gaseous streams such as those containing natural gas, industrial gases, and digester gas. Most of these processes relate to the removal of hydrogen sulfide. Because of its substantial acidic character, hydrogen sulfide is readily absorbed by alkaline absorbents. Unfortunately, the same absorbents also retain carbon dioxide, and they are not generally satisfactory for removing a number of other important industrial by-product sulfur compounds such as mercaptans, COS, and other "Total Reduced Sulfur compounds".
The term "Total Reduced Sulfur compounds" herein refers in general to sulfur compounds with the redox state of the sulfur atom at the same or a more reduced state than that of an elemental sulfur, it and includes, inter alia, H.sub.2 S, COS, CS.sub.2, mercaptans such as methyl mercaptan, butyl mercaptan and the like, organic sulfides and disulfides such as dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, thiophene, and similar materials including homologs of foregoing.
U.S. Pat. No. 4,822,576, issued Apr. 18, 1989, to H. H. Chu, discloses a process for removing hydrogen sulfide from a carbon dioxide containing gas mixture by solvent extraction with a tertiary amine.
U.S. Pat. No. 5,147,620, issued Sep. 15, 1992, to Linko Enterprises, discloses a process for removing hydrogen sulfide from natural gas by passing through an aqueous solution of copper (II) ions.
U.S. Pat. No. 4,880,609, issued Nov. 14, 1989, to Champion Chemicals, discloses a process for removing hydrogen sulfide from a stream of natural gas by contacting with sodium nitrite.
U.S. Pat. No. 4,892,718, issued Nov. 12, 1985, discloses a process for decontaminating COS, CS.sub.2, mercaptan, H.sub.2 S and SO.sub.x from gases by scrubbing using a solution of a salt of scandium, yttrium, an element of the Lanthanide or Actinium groups or blends thereof.
U.S. Pat. No. 4,824,645, issued Apr. 25, 1989, to Mobil Oil, discloses a process for removing hydrogen sulfide from a gas stream from a natural gas production well containing carbon dioxide by contacting with a polyvalent metal chelate solution such as iron-HEDTA (N-(2-hydroxyethyl)ethylenediaminetriacetic acid) solution.
Unfortunately, the chemicals used in most of the-above mentioned processes have problems of being environmentally undesirable, chemically unstable or not regenerable during the process. For instance, tertiary amine itself is an odorous material and is thus environmentally undesirable. Copper is potentially toxic. Nitrite is a pollutant. The chelating agent HEDTA in an iron-HEDTA solution often decomposes during the process and replenishing the chelating agent is costly.
There is a line of patents disclosing the use of manganese dioxide for removing sulfur contaminants from gas stream. Manganese oxides are non-toxic and are extremely insoluble in water. Therefore, process utilizing manganese oxides have less environmental disposal problems than those materials used in other prior art processes.
Chinese patent application No. 1,052,260 published Jun. 19, 1991, discloses a process for treating sulfur oxides using a soft manganese ore containing about 60% MnO.sub.2. The Mn(IV) is reduced to Mn(II) and MnSO.sub.4 is recovered as a byproduct. The reference only discloses the removal of sulfur oxides and does not teach the removal of Total Reduced Sulfur. Moreover, no manganese regeneration is mentioned. A similar process is disclosed in Japanese patent application No. 3,207,427, published Sep. 10, 1991.
U.S. Pat. No. 4,552,735 and U.S. Pat. No. 4,552,734, both issued Nov. 12, 1985, disclose processes for removing Total Reduced Sulfur compounds from industrial gases by absorption and/or oxidation on dried manganese dioxide particles which contain an alkaline material. The spent manganese oxide particles are collected by an electrostatic precipitator or a bag filter unit or a bag-house. Since the manganese dioxide particles utilized are in dry form, these processes are less efficient than those processes utilizing aqueous slurries of manganese oxides.
U.S. Pat. No. 4,713,225, issued Dec. 15, 1987, discloses a process for removing organic reduced sulfur compounds using pre-fabricated foraminous filters containing manganese dioxide. The preparation and the regeneration of the active manganese dioxide containing filters are time consuming. Moreover, the process is a dry process and is therefore less efficient than a wet process.
U.S. Pat. No. 4,923,688, issued May 8, 1990 to J. Iannicelli, discloses a wet scrubbing process for removing Total Reduced Sulfur compounds using activated manganese dioxide absorbent particles which may contain alkaline material such as sodium hydroxide, sodium hydrogen carbonate and sodium carbonate. However, wet scrubbing processes using manganese oxides under alkaline conditions are often not efficient for the removal of certain reduced sulfur compounds. Moreover, an alkaline scrubbing solution also absorbs carbon dioxide and forms manganous carbonate precipitates which is difficult to be regenerated back to MnO.sub.2, Mn.sub.3 O.sub.4 and Mn.sub.2 O.sub.3.
These prior art processes utilizing MnO.sub.2 are not efficient and are therefore costly. Since the 1990 Amendment to the Clean Air Act requires more facilities to have air toxics control, there is a need for an improved process which can efficiently remove Total Reduced Sulfur such as H.sub.2 S and mercaptans and which provides a simple and economic method for the regeneration of the chemicals used.